In-situ TEM Study of the Liquid-Phase Reaction of Ag Nanowires with a Sulfur Solution

Using transmission electron microscopy (TEM) to directly observe a dynamic chemical reaction process in liquid-phase environment is a big challenge because it is difficult to keep liquid reactants under vacuum. In this work, we reported a liquid-based cell that enables in-situ observation of a solution reaction process under TEM. The novel liquid cell design not only realizes the self-alignment of top/bottom windows, but also achieves an adjustable liquid layer thickness down to nanometer scale. The cell consists of top and bottom frames, both of which are fabricated from silicon wafers using conventional micro-fabrication techniques. The transparent observation windows are made of silicon nitride (SiNx) membranes. In a typical assembling process, an ethanol solution containing silver nanowires and an ethanol solution containing saturated sulfur were sequentially dropped into the liquid tank of the bottom-frame of a liquid cell by using 1 mL syringe. Then, the liquid tank was covered by the top-frame with a window direction of 90 degrees, and an epoxy was used to seal the edge between the top-frame and bottom-frame. Using this assembled liquid cell, we performed in-situ TEM observation of the chemical reaction between Ag nanowires and sulfur in ethanol solution. Additional ex-situ X-ray diffraction (XRD) and Raman spectroscopy were also performed to study the reaction intermediates and final products. Instead of a simple reaction process in which sulfur diffuses into Ag to form the final product of Ag2S, we found that the real reaction process involves the formation of a soluble intermediate phase (Ag2S4), which led to a partial dissolution of Ag nanowires in ethanol solution during reaction. These results well demonstrate that the in-situ TEM technique is a powerful tool to reveal the "real" chemical reaction mechanism. The experimental technique developed here could also be used to study a broad range of dynamic phenomena in liquid environment.